'unsweet' originates from Old English components: the negative prefix 'un-' (from Old English 'un-') combined with 'sweet' (Old English 'swete').

'sweet' changed from Old English 'swete' (related to Proto-Germanic *swōtaz/*swōt-) and became Middle English 'swete' before the modern English 'sweet'. The negative prefix 'un-' has existed in Old English as 'un-' and has been used to form antonyms.

Initially, the components simply expressed the negative of 'sweet' ('not sweet'); over time the combined form 'unsweet' has continued to mean 'not sweet' or 'not sweetened' in modern usage.